In an industrial irradiation process using high energy electrons (5-10 MeV) dose uniformity throughout a product is an important requirement. A high uniformity and hence low "max/min ratio" allows the entire product volume to receive the minimum required dose with few areas of overdose, where damage to the product may result. Maintaining a uniform absorbed dose just above the minimum required also results in maximum economic use of the delivered beam power.
Maintaining dose uniformity in a homogeneous medium is relatively simple using a uniform scanned beam through which the product is conveyed. However, significant variations in the effective thickness and density of packages that are presented to the irradiation beam can be expected to occur with non-homogeneous products. These variations can be minimized or highly accentuated depending upon the manner in which the materials are packaged.
It is known that a non-uniform dose distribution may be achieved with a non-linear scan-magnet drive waveform for simple product geometry variations. Other common methods of varying the absorbed dose across a product include the use of beam absorbing masks in front of the product, beam deflectors and the use of multiple beams, either from the same or different sources, directed onto different areas or sides of the product. Scan drive waveform manipulation is limited by the electrical characteristics of the magnet and the beamline magnetic optics. Masking and deflector techniques require the use of mechanical devices and a means of changing these devices, either manually or remotely, for different product configurations. In general, these methods depend on manipulation of the beam as delivered from the source, offer coarse adjustment to the dose field map and may result in poor beam use efficiency, particularly in the case of masking. In addition cooling of masks and deflectors is often required for high power beams.
Attempts have been made to vary average beam power delivered to a target by beam modulation. In U.S. Pat. No. 4,457,803 Takigawa, there is described a method for controlling a focused ion beam for etching micropatterns on silicon wafers for producing semiconductors. Etching depth is adjusted by controlling etching time by applying a variable blanking signal to blanking electrodes adapted to turn off the ion beam. In U.S. Pat. No. 4,551,606 Inoue, there is described a similar method of controlling beam power dissipated in a workpiece by controlling the width of a group of uniform pulses. Both Takigawa and Inoue deal with relatively low particle energy beams that are used for surface effects only. The techniques described are used with direct current (dc) machines in which the beam is simply turned on and off at the source. The beam power absorbed is a function of dwell time of the beam at a particular location on the target which is located inside the beam source vacuum envelope.
A practical industrial irradiator must produce a beam of high energy particles for deep penetration and of sufficient current density (power) to process large volumes of product. A high power linear accelerator (linac) is a known method of achieving such beams. A linac consists of a series of resonant cavities in which microwave power is used to establish an electromagnetic field that imparts energy to the particle beam as it traverses the cavities. The resonant frequency of the cavities is influenced by the beam and requires the beam for efficient coupling of rf power to the accelerator structure. Accordingly, the solution of simply switching on and off the source as described in Takigawa and Inoue is not suitable to high power, high energy linear accelerators.